How do jellyfish function without a brain, heart, blood etc. They can hunt, defend themselves and move around, but it would appear like they cannot 'think' at all, at any level. They seem unlike any other animal, and yet get on quite well, how?

Well James, actually this is a very complicated discussion you have started. As you know(or don't know ) jellyfish are part of philum cnidaria. At the most primitive cnidaria(hidrozoars) the nervous sistem is called a diffuse nervous sistem: it has multipolar neurons on the surface of it's body()kinda resembles the Meisner and Auerbach plexes of the stomach). here we find the simplest reflex in the animal kingdom. The nervous system in jellyfish(scifozoars) is more complex because it is concentrated more on the region of the umbrella. So they do not a brain, but they have brain cells and they are organised well enough to fulfill the needs of these creatures...
Now, blood and heart. Simple: jellyfish are diploblasts. They have only 2 layers of cells. The digestion takes place in the gastral cavity and the food gets absorbed through there. The nutrients get to the exterior layer through diffusion. So they do not need blood or a heart.
Anything else? I am "in the driving seat" now Regards,
Andrew

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

Another fascinating fact about jellyfish is that they don’t eyes. Some jellyfish have ways of detecting obstacles that can be compared to sight but they don’t have real eyes.

It is a mystery how they can process the information from their “sight” since they doesn’t have any brain. They react directly on food and danger stimuli via nerve impulses without having any brain to process the impulses. Another fact about jellyfish and their bodies is that the bodies are over 95% water.